Ferrous base alloy



United States Patent FERROUS BASE ALLOY Robert F. Thomson, Grosse Pointe Woods, and James C. Holzwarth, Birmingham, Mich., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware No Drawing. Original application July 5, 1955, Serial No. 520,149. Divided and this application March 31, 1958, Serial No. 724,891

2 Claims. (Cl. 75-122) This invention relates to an improved zinc base alloy and a process for producing such an alloy. More particularly, the invention pertains to an alloy of this type which is characterized by outstanding wear resistance properties due to the presence of hard particles of an iron-zirconium-manganese alloy. The present application is a division of United States patent application Serial No. 520,149 which was filed on July 5, 1955.

Zinc base alloys commercially used today for drawing dies and similar purposes usually possess inadequate wear properties for many requirements. It is therefore a principal object of the present invention to overcome this deficiency by providing a zinc base alloy characterized by greatly increased wear properties, high resistance to fracture, good castability and homogeneity. It is a further object of this invention to provide a drawing die formed of an inexpensive zinc base alloy which possesses high Wear resistance, a low melting point and uniform shrinkage.

These and other objects and advantages are attained in accordance with the present invention with a zinc base alloy containing a small amount of dispersed particles of iron-zirconium-manganese alloy. These particles contain hard intermetallic compounds or phases of manganesezirconium which are primarily responsible for the outstanding wear resistance of the zinc base alloy.

In particular, we have found that a zinc base alloy which contains small amounts of copper and aluminum, as well as the aforementioned hard particles of iron-zirconium manganese, is especially suitable for use as a drawing die. A small amount of magnesium also may be advantageously included in the alloy. In this type of zinc base alloy the aluminum and copper are added to increase the tensile strength and hardness and to reduce the solidification temperature of the alloy. Magnesium is preferably included in the alloy to overcome the corrosive influence of any impurities which may be present in the alloy. It therefore promotes dimensional stability and prevents a decrease in the strength of the alloy on aging. The resultant material is a long-wearing, generally homogenerous alloy having good castability properties.

As disclosed in United States Patent No. 2,720,459, the wear resistance of a zinc base alloy may be improved by the inclusion of dispersed, hard particles of nickeltitanium in the alloy. However, iron-zirconium-manganese alloys do not contain any relatively critical nickel and hence are preferred in the event of a nickel shortage because of a national emergency or for other reasons. Furthermore, the iron-zirconitun-manganese alloy is less expensive than typical nickel-titanium alloys.

In accordance with the invention, therefore, the aforementioned beneficial properties are obtained to a particularly high degree in a zinc base alloy containing aluminum, copper and magnesium by the inclusion therein of iron-Zirconium-manganese in the form of small particles which are generally uniformly dispersed throughout the zinc base alloy. As hereinafter more fully explained, these particles may be advantageously introduced into the zinc-rich melt in the form of a copper base intermediate alloy.

The low melting point of the zinc base alloy eliminates the need for elaborate equipment in the alloying procedure, only a comparatively simple gasor oil-fired melt: ing kettle being required. The uniform shrinkage characteristic of this alloy permits the exact size of castings to be predetermined with precision, eliminating the necessity for extensive use of profiling machines. Accordingly, the subject alloy is ideally suited for use as a drawing die material since the processing of dies formed of this alloy is comparatively simple, requiring a minimum of equipment and labor. Cost is further reduced by the fact that this alloy can be remelted many times, permitting the material in obsolete dies to be almost entirely recovered.

In addition, the facility with which this Zinc base alloy can be cast and the diversity of forms into which the molten alloy will flow make it a very desirable material for a variety of purposes. Furthermore, finished castings of this alloy can be produced comparatively quickly, making them available for production use within a relatively short time.

The high wear resistance of the final zinc base alloy is due to the presence of the dispersed, hard particles of ironzirconium-manganese alloy in the softer matrix. These particles do not readily float or settle out of the zinc-rich melt since they have a specific gravity which closely approximates that of the melt, provided the iron content is not excessive. Hence the present invention provides an alloy which as proper particle distribution, as well as optimum particle size, resulting in physical characteristics which satisfy all requirements for an outstanding tool alloy.

Commercially satisfactory results may be obtained in accordance with our invention with a final zinc base alloy containing approximately 1% t0 3% by weight of hard particles of iron-zirconium-manganese alloy. However, the iron-zirconium-manganese particles may be present in amounts as large as about 4% by weight, and in some instances a noticeable improvement in wear resistance results when these particles constitute as little as about 0.5% of the zinc base alloy. If the alloy contains more than approximately 4% of these particles, the castability of the alloy is impaired and its cost becomes excessive. Hence a desirable zinc base drawing die alloy which possesses exceptional wear resistance is one consisting essentially of about 2% to 5% aluminum, 0.5% to 5% copper, 1% to 3% hard particles of iron-zirconiummanganese and the balance substantially all zinc. The inclusion of approximately 0.02% to 0.3% magnesium is beneficial to reduce the corrosive tendencies of impurities such as lead, cadmium and tin. It will be understood, of course, that the Zinc base alloy also may contain small amounts of silicon and other elements as incidental impurities.

Thus it can be seen that in accordance with our invention a zinc base alloy containing at least approximately zinc has its wear resistance appreciably improved by the presence of the aforementioned hard iron-zirconium manganese particles. It will be understood, however, that the term zinc base alloy, as used herein, is intended to encompass those alloys in which zinc is the major constituent and preferably constitutes at least 50% of the alloy.

More specifically, we have obtained outstanding wear characteristics in a cast alloy consisting essentially of 87% to 93% Zinc, 3% to 5% aluminum, 2% to 3.5% copper, 0.05% to 0.2% magnesium, and 1% to 3% ironzirconium-manganese alloy in the form of dispersed hard particles. An alloy consisting of approximately 4% aluminum, 0.15% magnesium, 3.25% copper, 1.5% ironzirconium-manganese and the balance zinc plus incidental impurities appears to possess optimum castability and wear resistance properties.

Wear resistance, of course, is a function of both the size and distribution of the hard iron-zirconium-manganese particles. Since particle size and distribution are dependent on such factors as metal viscosity, solidification rates and methods of alloying, this invention also provides a preferred procedure for preparing the zinc base alloy. In the case of a drawing die it is desirable to produce maximum wear resistance without causing scoring of the part being drawn.

The iron-zirconium-manganese alloy can be initially prepared by melting together the three individual constituents. Commercially pure zirconium and manganese may be conveniently employed. The resultant pre-alloy also may contain small quantities of other metals, such as aluminum, silicon, chromium, magnesium and nickel. Normally the maximum amounts of these latter metals would not exceed approximately 5% aluminum, 2% silicon, 1% chromium, 1% magnesium and 0.5% nickel. These exact percentages of the minor constituents are not critical in most instances, however, and are listed as examples only. Inasmuch as the iron-zirconium-manganese pre-alloy does not readily dissolve in the zinc-rich melt, it is preferred to introduce these hard particles by means of an intermediate alloy or hardener containing copper. When the iron-zirconium-manganese is added to molten copper, it is transformed substantially into the molten state. During solidification of the intermediate alloy, there appears to be a decrease in the solubility of the iron-zirconiummanganese alloy in the copper, and this alloy is therefore preferentially isolated as a network in the copper-rich matrix. In order to form long-wearing particles of suitable size, this hardener is preferably added to the zinc in the solid state. Since it is desirable to cast the copper base intermediate alloy in shapes in which the copper-rich matrix will dissolve most readily in the molten zinc-rich alloy, it is preferred to form castings having a high ratio of surface area to volume, such as fiat pieces or thin sheets. Generally the formed iron-zirconium-manganese particles have diameters in the order of about 0.001 inch. If the particles are much smaller than this, the wear resistance of the final zinc base alloy is not increased the desired extent.

Upon introduction of the intermediate alloy to the zincrich melt, the copper or copper-rich matrix is dissolved, leaving the relatively insoluble network of iron-zirconiummanganese suspended in the zinc as Wear-resistant particles of appropriate size. Agitation of the zinc-rich melt causes these particles to become generally uniformly dispersed through the melt, and the particles remain so dispersed in the solidified zinc base casting.

The preferred drawing die alloy composition may be obtained by melting substantially pure zinc and, after elevating the temperature of the molten zinc to between about 950 F. and 1075 F., dissolving therein the appropriate amount of aluminum. This addition of aluminum retards dressing of the zinc at higher temperatures and, if a cast iron melting pot is employed, it inhibits attack of the pot by the zinc-rich melt. After further raising the temperature of the melt to between approximately 1100" F. and 1300 F., the copper-iron-zirconiummanganese intermediate alloy is added in the solid state, as hereinbefore indicated. The elevated temperature should be maintained until the aforementioned copperrich matrix in this intermediate alloy is entirely dissolved, the solution rate being increased by periodic agitation. After this solution is accomplished, we have found it desirable to lower the temperature of the melt to approximately 900 F. to 950 F. A suitable flux, such as ammonium chloride, may then be added to remove dross from the melt. The magnesium is thereafter introduced, if it is to be included in the alloy, preferably by sub- 4 merging it in the bath. When the magnesium is dissolved, the final alloy may be cast to shape in suitable molds.

Although the aluminum can be introduced into the melt either before or after addition of the intermediate alloy, the above alloying sequence has been found to be most satisfactory. Alternatively, a portion of this aluminum may be added prior to the introduction of the intermediate alloy and the remaining aluminum added after this alloy addition.

As hereinbefore explained, it is desirable that the ironzirconium-manganese alloy have a density which approximates that of the zinc-rich melt in order to prevent floatation or segregation of the iron-zirconium-manganese particles. The density of zinc at its melting point is 6.92 grams per cc., and the addition of about 4% aluminum and 3.25% copper changes the density of the resultant alloy to slightly over 6.9 grams per cc. Therefore, in order to obtain proper distribution of the iron-zirconiummanganese particles, it is desirable to form these particles of an alloy having a specific gravity of about 6.5 to 7.5 grams per cc. The density of iron is 7.87 grams per cc. and the density of manganese is 7.2 grams per cc., while the density of commercially available zirconium is about 6.5 grams per cc. Hence, we have found that an iron-zirconium-manganese alloy consisting essentially of 50% iron, 25% zirconium and 25% manganese provides excellent results. Such an alloy has a calculated desity of about 7.43 grams per cc., which is slightly higher than the specific gravity of the aforementionad zinc-rich melt.

It will be noted, however, that the densities of zirconium and manganese permit the reduction or substantial elimination of iron in the pre-alloy insofar as the density requirement is concerned. For example, an alloy of 50% manganese and 50% zirconium has a calculated density of approximately 6.87 grams per cc. and is sufficiently dense so that the particles thereof will not present a serious floatation problem. Hence, the present invention in its broader aspects is intended to encompass the use of zirconium-manganese particles as well as iron-zirconium-manganese particles. Generally, however, it is much preferred to include iron in the pre-alloy since this reduces the cost of the resultant material to an appreciable extent and conserves the use of relatively scarce manganese and zirconium. Thus, the pre-alloy preferably contains at least 30% iron, and the iron may constitute as much as approximately 70% of the iron-zirconiummanganese alloy. I

We prefer to have a ratio of zirconium to manganese in the iron-zirconium-manganese alloy of about 1 to 1, but an appreciable variation in the relative amounts of these three constituents is permissible. Thus we have found that the wear resistance of a zinc base alloy may be substantially improved with an iron-zirconium-manganese pre-alloy comprising approximately 20% to 35% zirconium, 20% to 35% manganese and the balance iron. Such a pre-alloy produces particles of optimum size and density. In some instances, however, the pre-alloy may contain as little as 15% or as much as 55% zirconium, and the manganese content likewise may vary from about 15 to 55%.

When the iron-zirconium-manganese pre-alloy is mixed with the molten copper, usually at a temperature of 2200 F. to 2700 F., it is preferred to form an intermediate alloy containing approximately 55% to copper. If this alloy has a copper content less than 55 it is difficult to place the copper-rich matrix of the copperiron-zirconium-manganese intermediate alloy in solution in the zinc-rich melt. Therefore, a copper base alloy comprising about 3% to 20% zirconium, 3% to 20% manganese, 5% to 35 iron and the balance copper is appropriate for use in carrying out the present invention. In order to provide a zinc base alloy with approximately 0.5% to 4% iron-zirconium-manganese particles, the intermediate alloy normally constitutes about 1% to 9% of the final alloy, depending on the composition of the pre-alloy, although 3% to 6.5% is preferred. When such an intermediate alloy is added to a zinc-rich melt, it should introduce into the final alloy approximately 0.2% to 2% iron, 0.15% to 1% zirconium and 0.15% to 1% manganese in the form of iron-zirconium-manganese particles and about 0.5% to 5% copper which is not combined with these particles.

Since the hard particles result principally from the combination of zirconium and manganese and are formed during the preparation of the pre-alloy, the allowing procedure employed in forming the hardener is of importance in achieving optimum results. Accordingly, the ironzirconium-manganese pre-alloy may be compounded by melting together the proper amounts of these elements,

. preferably at a temperature of approximately 2900 F.

to 3100 F. Inasmuch as zirconium is a rather readily oxidizable and nitridable element, it is desirable to use an inert gas as the melting atmosphere. We have obtained most satisfactory melting and high zirconium recovery using an induction furnace under an argon atmosphere.

It will be noted that it is necessary to form particles of manganese-zirconium or iron-zirconium-manganese in order to obtain high wear and score resistance in accordance with the invention. Merely adding the iron, zirconium and manganese separately to the zinc-rich melt, even if these constituents are introduced in the aforementioned preferred proportions, does not form these hard particles or provide the necessary wear resistance. It is the alloy of iron, zirconium and manganese, rather than the individual elements, which contributes the desirable properties of wear and score resistance to the final zinc base alloy.

Wear tests were conducted to compare zinc base alloys formed in accordance with our invention with the same material devoid of iron-zirconium-manganesc particles. Samples 1% inches wide and as inch high were prepared from the cast zinc base alloys to be tested, and each speci' men was machined at one edge to prepare a .4: inch by 1 inch rubbing surface. The specimens were next we cessively locked in a fixture of a wear test machine and placed in contact with a rotating smooth-surfaced wheel of low carbon steel having a face width of one inch. Increased wear resistance was measured by decreased weight loss in grams.

A wear test using this apparatus was conducted in which the specimen load was increased during a five-hour period from zero load and automatically adjusted to produce a constant frictional load rather than a constant load normal to the wheel. This test included a ten minute run-in period in which only the weight of the specimen being tested and its holder bore against the wheel, a period of 1 /4 hours to load the specimen to 500 pounds, a 30 minute period at 500 pounds to establish the frictional characteristics, and the balance of the five hours run with this established value of friction maintained constant. After each test any loosely adhering, deformed metal and burrs were removed from the wear test sample, and loss in weight values were used in comparing the wear resistance of the specimens.

At the end of the test period the zinc base alloy speci mens formed from a zinc base alloy consisting essentially of 3.25% copper, 4% aluminum, 0.1% magnesium and the balance zinc showed an average weight loss of 0.4764 gram. On the other hand, a zinc base die alloy specimen of similar composition but containing the aforementioned preferred amounts of the iron-zirconium-manganese particles lost an average of only approximately 0.0248 gram. The results of this test show how greatly the presence of dispersed particles of the hard iron-zirconium-manganese alloy increases the wear resistance of zinc base alloys.

Although the final alloy formed has been described as particularly suitable as a drawing die material, it also may be employed to considerable advantage in other applications in which high wear resistance, good castability, uniformity of properties throughout a cast section, good machinability, and anti-score properties are of importance.

While we have set forth herein specific examples of zinc base alloys possessing high wear resistance characteristics due to the presence of hard particles of manganese-zirconium or iron-zirconium-manganese, it is not intended to restrict the invention to any specific zinc base alloy. We believe that we are the first to discover the value of adding these particles to zinc base alloys generally, and the in vention is not to be restricted except as defined in the following claims.

We claim:

1. A ferrous alloy consisting essentially of 30% to 70% iron, 15% to zirconium and 15% to 55% manganese.

2. A pre-alloy for addition to a zinc base alloy, said pre-alloy consisting essentially of 20% to 35% zirconium, 20% to 35% manganese and the balance substantially all iron.

No references cited. 

1. A FERROUS ALLOY CONSISTING ESSENTIALLY OF 30% TO 70% IRON, 15% TO 55% ZIRCONIUM AND 15% TO 55% MANGANESE. 